The present invention relates broadly to a method of heat treating a silica start rod by a plasma and, more specifically, to an improved method of using heat of a plasma for finishing silica start rods of the type used in a soot deposition technique.
It is recognized generally that optical fibers are a superior medium of communication. For example, a single fiber can carry hundreds of times more information than a simple metal wire can. However, fiber optic systems have yet to approach the cost-performance characteristics of metal systems. For fiber optical technology to become successful commercially it must be produced at a competitive price. Therefore, it is important from a cost consideration standpoint to produce relatively large quantities at relatively low costs.
Aside from but related to production cost considerations, is the critically of minimizing optical transmission losses to comercially acceptable levels.
Transmission losses or attenuation of light is due primarily to impurities causing scattering and/or absorbing of such light. Optical purity is extremely difficult to attain especially considering the fact that mminute quantities of impurities, for instance in the order of several parts per million, can add significantly to transmission losses. This possibility for contamination is compounded when it is considered that several steps are typically involved with the fabrication of optical fibers.
One major technique for fabricating optical fibers in an economical and mass production fashion is the so-called "soot deposition" process. Typically, in this process, glass precursor vapors are introduced into a hydrolyzing flame. The result is formation of adherent particulate material (i.e. soot) which is directed towards a rotating and translating start-up material upon which the soot adheres to form a soot preform. Ordinarily, the start-up mandrel is fabricated from fused silica or doped fused silica. Following deposition, the soot preform is dehydrated and then consolidated into transparent fused silica. Subsequently both the mandrel and consolidated preform are drawn, whereby the mandrel becomes the core and the consolidated preform becomes the cladding. From a commerical standpoint this approach is highly desirable since the deposition rates are generally rapid. For example, a soot preform capable of forming 20 kilometers of optical fiber may be prepared in a few hours.
One area of potential transmission losses is the interface between the start rod and the preform. This is due primarily to surface defects of the type that include particulate, cracks and scratches on the start rod. Even microscopic defects of the type noted can be troublesome. Such defects are difficult to avoid in the normal handling of such start rods, let alone are capable of easy detection. For minimizing transmission losses it is extremely important to have the surface of the start rod free of contaminants and essentially optically smooth.
A known approach for cleaning and smoothing the start rod is to etch the surface followed by a fire polishing technique. The etching removes the particulate contamination while the fire polishing causes the surface to soften which then allows inherent surface tension to facilitate smoothing of the external surface. However, by virtue of known fire polishing techniques impurities are still formed by reason of the combustion process caused by use of a conventional gas torch. A primary problem arising from conventional fire polishing is introduction of water into the start rod. By water it is meant OH, H.sub.2 and H.sub.2 O. Such water leads to light absorption peaks or zones in the fiber and contributes significantly to transmission losses.